Successful carbohydrate recovery from lignocellulosic biomass requires breaking intermolecular bonds in glucan and xylan chains while avoiding further reaction of the resulting glucose and xylose. However, in neutral or dilute aqueous acid solutions (<10 wt % mineral acid systems), the resulting glucose further reacts to yield furans or other degradation products. The glucose degradation reactions significantly outpace cellulose depolymerization at temperatures below 250-350° C. depending on the acid content. This leads to the need for reaction systems with short residence times (10 ms to 1 min) at high temperatures (250-400° C.) in order to obtain a high selectivity to glucose at high conversion, while simultaneously minimizing degradation of the desired glucose product. See Bobleter (1994) Prog. Polym. Sci. 19:797-841 and Peterson et al. (2008) Energy Environ. Sci. 1:32-65. These types of short-residence-time reactions are especially impractical when using heterogeneous starting products such as biomass. Higher yields are obtainable at lower temperatures and longer residence times using increased homogeneous catalyst concentrations such as high mineral acid concentrations and/or ionic liquids. However, in both cases, the homogeneous catalyst and ionic liquid are a significant expense. Thus, recovering the catalyst and/or ionic liquid is critical for the commercial viability of these processes. Ultimately, recovering and recycling these materials end up being a significant component of the processing costs. See, for example, Von Sivers & Zacchi (1995) Bioresour. Technol. 51:43-52; Binder & Raines (2010) Proc. Natl. Acad. Sci. U.S.A. 107:4516-4521 (2010); and Shill et al (2011) Biotechnology and Bioengineering 108, 511-520 (2011).
Cellulase enzymes operating at only 50° C. can achieve near complete conversion of cellulose. However, in these processes, the cellulose must be rendered accessible by a thermochemical pretreatment of the raw cellulosic feed stock. Both enzyme and pretreatment costs are significant obstacles toward the successful commercialization of these processes. For example, enzyme costs are consistently shown to account for between US $0.50 and $2.00 per gallon of ethanol (2009 dollars). This expense is a significant portion of the overall cost of production. See, for example, MacLean & Spatari (2009) Environ. Res. Lett. 4(1):014001 and Wilson (2009) Curr. Opin. Biotechnol. 20(3):295-299.
Strategies have been developed to produce glucose and xylose from biomass while avoiding further degradation despite using low catalyst concentration and low temperature. One such strategy involves passing a solvent through a heated packed bed of biomass in a flow-through reaction system. This approach decouples the residence times of the solid carbohydrate polymer from its soluble counterpart. These systems are typically limited by their ability to produce reasonably concentrated product solutions. Indeed, using an aqueous solution of 1 wt % H2SO4 as the extraction solvent, glucose yields of only 45-55% can be achieved when using a 2-4 wt % sugar solution as the feedstock. See Lee et al. in “Recent Progress in Bioconversion of Lignocellulosics,” Chapter 10 (Dilute-Acid Hydrolysis of Lignocellulosic Biomass), pp. 93-115, © 1999 Springer-Verlag Press, Berlin, Germany, ISBN: 978-3-540-65577-0; and Liu & Wyman (2004) Ind. Eng. Chem. Res. 43:2781-2788.
In recent work, GVL-water solutions coupled with very dilute acid concentrations (>0.1 M H2SO4) or solid acid catalysts have shown the ability to solubilize lignocellulosic biomass and promote dehydration of glucose to levulinic acid and of xylose to furfural. See Gürbüz et al. (2013) Angew. Chem. Int. Ed. 52:1270-1274; Alonso et al. (2013) Energy Environ. Sci. 6:76-80; and Wettstein et al. (2012) Energy Environ. Sci. 5:8199-8203.
Published U.S. Pat. Appl. US 2014/0 194 619, to Dumesic et al., published Jul. 10, 2014, describes a process to produce an aqueous solution of carbohydrates (C5- and C6-sugar monomers and oligomers) from biomass by reacting the biomass with a solvent system comprising a beta-, gamma-, or delta-lactone and at least 1 wt % water, in the presence of an acid catalyst. This process results in a product mixture containing water-soluble C6-sugar-containing oligomers, C6-sugar monomers, C5-sugar-containing oligomers, and/or C5-sugar monomers. A solute is added to the reaction mixture in an amount sufficient to cause partitioning of the mixture into an aqueous layer and a substantially immiscible organic layer. This process, while effective, is not necessarily economically efficient. For example, the lactone solvent (typically gamma-valerolactone, GVL) must be recycled for the process to be economically feasible. This separation can be done very efficiently using liquid carbon dioxide. However, extracting with liquid carbon dioxide requires high-pressure equipment—an added capital expense. Additionally, the lignin present in the raw biomass is solubilized and retained in the CO2-expanded lactone phase. Thus, the lactone solvent must be recovered from the lignin. This recovery requires evaporating the relatively high boiling point lactone from the lignin. The resulting lignin fraction is recalcitrant to any further processing. It is typically sold as low-value fuel.
Thus, there remains a long-felt and unmet need for an economically efficient process to produce C6-sugar-containing oligomers, C6-sugar monomers, C5-sugar-containing oligomers, C5-sugar monomers, and/or other dehydration products from biomass and/or a biomass-derived reactant.